1. Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) switching network and more particularly to a redundancy structure for use when a fault is found in the switching network.
The present invention is intended to provide an economical redundancy structure which is based on the idea of making a call bypass a faulty switch through an alternate route including a switch which belongs to the same switching stage as the faulty switch in an asynchronous transfer mode switching network.
2. Description of the Related Art
Although no redundancy structure for an ATM switching network has been made known before, conventional redundancy structures for electronic switching systems consist of two switching networks analogous to each other and are submitted to dual operation: one is used as the acting system while the other is disconnected and kept for use as a standby system. When trouble occurs in the acting system, the standby switching network is connected and the acting system is disconnected, thus maintaining the connections of lines through the switching system. The dual operation of the redundancy structure has been referred to in "D70 Model Automatic Switching System" edited by NTT (Nippon Telephone and Telegraph Co., Ltd.).
FIG. 1(a) is a block diagram of a redundancy structure of a conventional switching network.
This switching network comprises two identical three-stage networks whose details are illustrated in FIG. 1(b), a 0-system switching network 20 and a 1-system switching network 21, switchover circuits 3 for choosing one of the switching networks, and a switchover control circuit 4 for directing the switchover operation. As illustrated in FIG. 1(b), each of the switching networks of the 0-system and the 1-system consist of eight unit switches in each stage of the three-stage networks. As one unit switch has eight incoming and eight outgoing terminals, eight incoming lines are connected to each of the eight unit switches 100, 101, . . . , 107, and eight outgoing lines are connected to each of eight unit switches 300, 301, . . . , 307.
Links between each adjoining stage are connected as follows:
Each of the eight links from outgoing terminals No. 0, 1, . . . , 7 of unit switch 100 of the first stage is connected to No. 0 incoming terminal of each unit switches 200, 201, . . . , 207 in the second stage, respectively.
Each of the eight links from outgoing terminals No. 0, 1, . . . , 7 of unit switch 101 of the first stage is connected to No. 1 incoming terminal of each unit switches 200, 201, . . . , 207 in the second stage, respectively.
In this manner, each of eight links from outgoing terminals No. 0, 1, . . . , 7 of unit switches 102, 103, . . . , 107 is connected to No. 2, 3, . . . , 7 incoming terminal of each unit switches 200, 201, . . . , 207 in the second stage, respectively.
As for links of unit switches 300, 301, . . . , 307 in the third stage, similar connection arrangement is provided to unit switches 200, 201, . . . , 207 in the second stage as shown in FIG. 1(b).
The 0-system switching network is used as the active system for communication, and the 1-system switching network remains as a standby system. When trouble occurs in one of the unit switches of the 0-system switching network 20, the switchover control circuit 4 gives the switchover circuit 3 a switchover command and all the incoming and outgoing lines are switched from the 0-system switching network 20 to the 1-system switching network 21. Fault detection is then carried out in the 0-system switching network. After the faulty switches have been replaced, the 0-system switching network 20 is restored for use.
The conventional electronic switching system thus necessitates a switchover circuit and switchover control circuit in addition to a double network which consists of a number of unit switches. The disadvantage is that such a switching system tends to be costly and bulky. Moreover, as the entire switching network is switched over even when only part of it malfunctions, all calls being communicated can be adversely affected. In other words, the disadvantage lies in the fact that a complicated series of circuits or software control is required to maintain continuity of any given call in areas other than the source of the problem during the switching over of the network.
In "A.T.M. switching architecture for broadband ISDN" at the IEEE International Conference on Communication '88 at Philadelphia, Pa. on Jun. 12-15, Messrs. Hajikano, Murakami, Iwabuchi, Isono, and Kobayashi disclosed a multi-stage self-routing switch network (MSSR) which can bypass a faulty unit switch in the second stage, but they did not disclose a redundancy structure, especially, parallel wiring to a plurality of unit switches at the first stage and final stage from each incoming line and outgoing line for selective connection nor did they disclose the increase in transmission speed of the link over the transmission speed of incoming and outgoing lines.